1. Field of the Invention
The present invention relates to a toner for developing latent electrostatic images in electrophotography, electrostatic recording, electrostatic printing or the like and also relates to a developer, a toner container, a process cartridge and an image forming method each using the toner.
2. Description of the Related Art
Generally, image formation by electrophotography, electrostatic recording, electrostatic printing etc. is performed by a series of processes in which a latent electrostatic image is formed on a latent electrostatic image bearing member (hereinafter, otherwise referred to as “photoconductor” or “electrophotographic photoconductor”), the latent electrostatic image is developed using a developer to form a visible image (toner image), the visible image is transferred onto a recording medium such as paper, and then fixed to form a fixed image on the recording medium.
As the developer, there are two types of developers, i.e. a one-component developer singularly using a magnetic toner or a non-magnetic toner, and a two-component developer composed of a toner and a carrier.
As a fixing method used in electrophotography, heat roller methods are widely and generally used in terms of increased energy efficiency, in which a heat roller is directly press-contacted with a toner image on a recording medium. The heat roller methods require a large amount of electric power for fixing. So, for the purpose of saving energy, a variety of studies have been done in an attempt to reduce electric power consumption of heat rollers. For instance, a method is generally and often used, in which at the time when it is not intended to form an image, the output power of a heater for heating the heat roller used is reduced, and at the time of outputting an image, the output power of the heater is increased to thereby increase the temperature of the heat roller.
In this case, however, in order to increase the temperature of the heat roller to a temperature necessary for fixing, from the temperature at sleep time, a standby time period of about several tens seconds is necessary, and users can be stressed from the standby time period. Alternatively, when it is not intended to form an image, it is desired to reduce the power consumption by fully turning off the heater. To achieve these demands, it is necessary to lower the fixing temperature of toner itself to decrease the fixing temperature of toner in available time.
With the development of electrophotographic technology, toner used in the developer is required to have excellent low-temperature fixability and storage stability (blocking resistance), and there have been a variety of attempts to use polyester resins which have higher affinity with recording media and the like and have more excellent low-temperature fixability than styrene resins which had been generally used as toner binder resins. For example, there have been proposed a toner containing a linear polyester resin whose physical properties, such as molecular weight, are specified (see Japanese Patent Application Laid-Open (JP-A) No. 2004-245854), a toner containing a non-linear crosslinkable 20 polyester resin using rosins as acid components (see Japanese Patent Application Laid-Open (JP-A) No. 4-70765, and the like.
In order to achieve further high-speed operation performance and energy saving in image forming apparatuses, conventionally used toner binder resins still leave a lot to be desired in terms of responding to market demands, and it is difficult to maintain sufficient fixing strength of toner by shortening the fixing time in a fixing step and/or lowering the heating temperature of a fixing unit.
A toner containing a polyester resin using rosins as i described in JP-A No. 4-70765 is superior in the low-temperature fixability as well as the pulverizability, and thus the toner is advantageous in improving the toner productivity by a pulverization method. Also, use of 1,2-propanediol, which is a branched-chain alcohol having 3 carbon atoms, as an alcohol component makes it possible to improve the low-temperature fixability while maintaining the offset resistance, as compared to use of alcohol having 2 or less carbon atoms, and is effective in preventing degradation of the storage stability accompanied by a reduction in glass transition temperature, as compared to a branched alcohol having 4 or more carbon atoms. The use of such a polyester resin as a toner binder resin makes it possible to provide such an effect that an image can be fixed at a low-temperature and the storage stability can be improved.
The demand for energy saving tends to be more and more increased, and the low-temperature fixability of toner has become more improved than before, by using polyester resins excellent in low-temperature fixability, however, it will be difficult in the near future to satisfactorily respond to the demand for energy saving by just using polyester resins.
In recent years, attempts have been made to improve the low-temperature fixability of toner by incorporation of a fixing-assisting component to the toner (see Japanese Patent Application Laid-Open (JP-A) No. 2006-208609). JP-A No. 2006-208609 proposes a toner which satisfies both heat-resistant storage stability and low-temperature fixability by allowing a fixing-assisting component to exist as crystal domains in the toner. With higher speed operation performance in recent years, it will be desired to satisfy the demands for high-durability and further energy saving of toner, and accordingly, further improvements and developments are still desired.